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Abstract

The structure of the B2 phase has been investigated in Ti-25Al-30Mo, Ti-25Al-35Mo and Ti-25Al-40Mo alloys using Rietveld refinement of X-ray diffraction data in homogenized condition. Different initial structure models have been used for the refinement based on the analyzed alloy chemistry. Site occupancy models for the general alloy compositions (xTi-yAl-zMo) wherein the Ti (x) is less than 50 atom % have been proposed. The site occupancy of the B2 phase has been calculated and compared with those of earlier experimental and theoretical investigations. The lattice parameter of the B2 phase decreases with increase in Mo content at constant Al.

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1. Introduction

The intermetallic compounds with B2 (ordered bcc, CsCl type) structure have been observed in several binary, ternary and multi-component alloys (Banerjee, 1993; Banerjee, 1997; Kim, 1989; Bradley & Taylor, 1937). The stoichiometric composition of this phase in binary alloy is AB while in ternary alloys is A2BC or A4B3C. However, these alloys often exist over a wide range of compositions on either side of the stoichiometric composition. The deviations from the stoichiometric composition are accommodated by constitutional defects namely vacancies on the different sublattice sites and/or antisites (Bradley & Taylor, 1937). The properties of the B2 phase change drastically with deviation from stoichiometric composition (Westbrook, 1967). This behavior is quite different from that of solid solution for which properties normally vary smoothly with compositions (Westbrook, 1967).

Site occupancy of the B2 phase can be experimentally determined by several competing or supplementary techniques and also can be predicted theoretically. This has been summarized by Jones in his review article on the determination of locations of chemical species in ordered compounds using atom location by channeling enhanced microanalysis (ALCHEMI) (Jones, 2003). Rietveld refinement of X-ray and neutron diffraction data is one of the techniques which has been used to determine site occupancy of the B2 phase for stoichiometric and non-stoichiometric alloys (Chaumat et al., 1999; Singh et al., 2007; Singh et al., 2008).

The B2 phase has been observed in Ti-Al-Mo system in a series of alloys which possess both the stoichiometric as well non-stoichiometric compositions either having single B2 phase or mixture of two or three phases (α2 + B2, α2 + γ + B2) (Bohm & Lohberg, 1958; Naka et al., 1993; Das & Das, 2003; Azad et al., 2006; Singh et al. 2007, 2008). Singh et al. (2007, 2008) have determined the site occupancy of the B2 phase in Ti-Al-Mo alloys using X-ray and neutron diffraction data. These results are classified into two groups: (1) the alloys containing Ti<50 atom % and (2) the alloys containing Ti>50 atom %. In former case, A sites are occupied by Ti atoms while the B sites are occupied by Al and Mo atoms. The small amount of Mo atoms can also occupy A sites either due to excess of B sites or anti site effect in non-stoichiometric alloys. In later case, the A sites are occupied by Ti atoms while the B sites are populated by Al and Mo atoms. The excess Ti atoms are occupied B sites.